The Effect Of Mineral Filler Coupling Plants On Wastewater Treatment Efficiency And Greenhouse Gas Emissions Of Constructed Wetlands

Constructed wetlands,as a cost-effective and efficient sustainable wastewater treatment technology,play a significant role in rural domestic wastewater treatment.However,the low carbon-to-nitrogen（C/N）ratio in rural domestic wastewater hinders the denitrification performance of traditional constructed wetlands.Additionally,the process of wastewater treatment can lead to the release of substantial amounts of greenhouse gases（CO₂,CH₄,and N₂O）,potentially shifting water pollution to air pollution and compromising the ecological value of constructed wetlands.To address these challenges,this study constructed a mineral media-assisted phytoremediation enhanced constructed wetland,optimizing key process parameters such as C/N ratio,plant species,and mineral media.The study also employed orthogonal experiments to determine the optimal operational conditions.The main findings of this research are summarized as follows.（1）Experimental investigations examined the combined effects of different factors（plants,media,C/N ratio）on pollutant removal efficiency.Nine sets of constructed wetland systems were evaluated for the removal rates of pollutants including NH₄⁺-N,total nitrogen（TN）,chemical oxygen demand（COD）,and total phosphorus（TP）.The removal rates ranged from 51.67%to 97.10%for NH₄⁺-N,35.31%to 85.77%for TN,71.78%to 92.12%for COD,and 78.93%to 92.25%for TP.The highest removal rate combination for NH₄⁺-N was achieved with reed and manganese sand at a C/N ratio of 5（A1B1C1）.For TN,the optimal combination was reed with mixed media at a C/N ratio of 8（A1B2C3）.Reed and manganese sand at a C/N ratio of 5（A1B1C1）resulted in the highest COD removal rate,while the highest TP removal rate was achieved with an iris plant and mixed media at a C/N ratio of 2（A3B3C1）.The range analysis indicated that the media had a significant impact on COD,NH4+-N,and TP removal rates,while the C/N ratio had a greater influence on TN removal rates,and the plant species had a relatively minor effect on pollutant removal rates.（2）Further experiments examined the combined effects of different factors（plants,media,C/N ratio）on methane（CH₄）,nitrous oxide（N₂O）emission reduction,and global warming potential（GWP）.The range analysis revealed that plants and C/N ratio had a significant impact on CH₄,N₂O,and GWP,while the media had a minor effect.The emissions fluxes of N₂O and CH₄ ranged from 1.11 to 2.73 mg/m²/d and 0.16 to 2.29 mg/m²/d,respectively,and the GWP ranged from 184.72 to 742 mg/m²/d.The combination of iris and manganese sand at a C/N ratio of 2（A3B1C1）resulted in the lowest CH₄ emissions,while the combination of iris and pyrite at a C/N ratio of 8（A3B2C3）led to the lowest N₂O emissions.The combination of iris and mixed media at a C/N ratio of 8（A3B3C3）resulted in the lowest GWP.（3）Scanning electron microscopy（SEM）results demonstrated the presence of fungal hyphae on the surface of the media.Furthermore,X-ray photoelectron spectroscopy indicated an increase in Mn（III）content from 10.90%to 13.60%in manganese sand and a decrease in Mn（IV）content from 28.41%to 20.84%.In the case of pyrite,besides the conversion of iron species between Fe（II）and Fe（III）oxidation states,various iron oxides or iron sulfides were formed.Additionally,during the transformation of sulfur into SO₄^2-,multiple valence states of sulfur intermediates(such as Sn^2-and S⁰)were generated.These electron transfer processes provide a theoretical foundation for elucidating the mechanisms of pollutant removal and greenhouse gas reduction.（4）Microbial distribution analysis of the enhanced phytoremediation wetland revealed that Nitrospira and other related genera were mainly associated with nitrification,while Denitratisoma,Dechloromonas,Azoarcus,Thiobacillus,Acidovorax,Candidatus＿Nitrotoga,Cupriavidus,unclassified＿k＿norank＿d＿Bacteria,and norank＿f＿JG30-KF-CM45 were the predominant functional genera associated with denitrification.Redundancy analysis indicated that the denitrification-related genera Denitratisoma,Dechloromonas,Thiobacillus,and Candidatus＿Nitrotoga showed positive correlations with pyrite media,while norank＿f＿JG30-KF-CM45 exhibited a positive correlation with manganese sand media.This suggests that the pyrite environment is more suitable for the growth of most denitrifying bacteria.